Air

[Cruizzin]

Why is there air?

[dinsdale]

Its the atmosphere of the planet, which is quite nice as without it there would not be much else in the way of land life.

[booksworth]

Think of it as God's ultimate challenge to the Chancellor of the Exchequer: if he can tax air, he can tax anything.

[New Forester]

Although we think of gases as weightless, they do of course have mass.  That means that any gas near a planet is attracted by its gravity, and gets piled up on its surface.  That is why larger planets all have atmospheres -- and why smaller ones, such as our moon, do not -- also why the density of an atmosphere is greater near the surface.

 

Our air came mainly from gases produced by volcanoes, which were previously dissolved in the molten rock.  Originally the atmosphere would have been very different from now -- methane, ammonia, nitrogen, carbon dioxide and probably quite a few more.  This atmosphere was what is called a reducing one -- there were no oxidising gases such as oxygen in it.  If you'd lit a match it would have gone out immediately.

 

This changed when plants first developed -- or at any rate, bacteria which could photosynthesise.  These used the sun's light to rip apart the huge amounts of carbon dioxide, make sugars, and release a potent environmental pollutant -- oxygen.  There are still bacteria now which cannot tolerate any oxygen, but most of life adapted, and the oxygen level increased more and more.  This made our type of energy production possible -- using oxygen to oxidise sugars to produce energy.

cont...

[New Forester]

...cont

Eventually photosynthesis used up all the carbon dioxide, and left us with an oxidising atmosphere containing a lot of oxygen (at times much more than now).  Most of the atmospheric carbon ended up deposited in limestone rocks, though some is in fossil peat, coal and oil, and of course in living things themselves.

By the way, remember that although we may talk of carbon dioxide as one of the normal atmospheric gases, it is extremely rare -- its natural concentration is only about a fiftieth of one percent, compared with around one percent for argon, twenty-one percent for oxygen and the rest nitrogen.  That is why burning of fossil fuels can so easily double the natural amount of carbon in the atmosphere.  It's really rather surprising that we've got away with it for so long.

[ezapf]

to breathe